The effects of cannabis use on neuropsychological indices that show characteristic disturbances in schizophrenia are unclear. The effect of cannabis use on these cognitive functions is of particular interest given the hypothesized association between cannabis use and schizophrenia. Therefore, this study aimed to examine the effects of cannabis use on attentional control, working memory and executive functioning, in both healthy individuals and patients with schizophrenia.

Neuropsychological performance was assessed in 36 cannabis users who were otherwise healthy, 35 healthy non-users, 22 cannabis-using patients with schizophrenia, and 49 non-using patients with schizophrenia. Participants were administered the Stroop task, the letter–number sequencing and spatial span subtests of the Wechsler Memory Scale, and the Wisconsin Card Sorting Test (WCST).

Patients with schizophrenia (both cannabis users and non-users) showed significantly poorer performance across all neuropsychological tasks, relative to controls; however, there were no significant differences between schizophrenic cannabis users and schizophrenic non-users on any measures, with the exception of increased non-perseverative errors on the WCST in cannabis-using patients. Similarly, healthy cannabis users showed no significant differences from healthy non-users in any of the cognitive domains, with the exception of a schizophrenic-like increase in perseveration on the WCST.

Amongst both healthy individuals and patients with schizophrenia there appears to be little difference in cognitive performance between cannabis users and non-users, suggesting that cannabis use has only subtle effects on the neurocognitive performance indices assessed here, which have been well established to be disturbed in schizophrenia.